What is Wastewater Treatment?

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Sewage treatment or domestic wastewater treatment is the process of removing contaminants from waste water and household sewage, both industrial effluents and domestic waste. It includes physical, chemical and biological processes to remove physical, chemical and biological contaminants. Its major goal is to produce an environmentally-safe fluid waste stream and a solid waste suitable for disposal or reuse (usually as farm fertilizer). Waste water is fresh water that has been used for industrial processes, for household cleaning, and in our bathrooms / toilets. It includes substances such as human waste, food scrapes, oils, soaps and chemicals. Wherever it comes from, the water is full of bacteria, chemicals and other contaminants.


Earth has been cleaning and recycling water for millions of years without any human intervention through the process of evaporation of ocean waters, cloud formation, and eventual condensation (rain). Also the bacteria and ground /dirt served as filters to take out solid waste from the water. In essence, the water we are drinking now is the same water that was consumed by Dinosaurs!! With the explosion of human population, and our propensity to live in dense clusters in the cities, it has become important for us to clean our water ourselves, as we cannot rely on mother nature do that for us. Hence the need for waste water treatment plants.


What are the different modes of waste water treatment?

In general use, there are 2 types of waste water treatment systems. They are as follows.

  • Biological waste water treatment plants: They use bacteria and other biological matter to break down the waste.
  • Physical /chemical waste water treatment plant: Industrial waste water can contain chemicals that can harm the ecosystem which creates the necessity for a chemical plant to treat this waste. Physical /chemical waste water treatment plants use both physical and chemical reactions to treat wastewater. The steps involved in the process are as follows.


What are the different steps involved in waste water treatment?

  • Primary Treatment: As a first step, waste water is screened to physically eliminate objects, like large solids using grates, screens and settling tanks. Some of the solid waste could wreck the treatment plant’s machinery. Next the wastewater is taken to a primary settling basin where matter can float or sink in the tank.
  • Secondary Treatment: The remaining water is then sent to the secondary treatment tank where the biological matter, such as bacteria, removes much of the left over suspended matter. These biodegradation processes also takes place in streams, lakes and oceans, but the purification systems in nature can easily be congested with contribution of too much organic waste. Secondary treatment prevents this type of pollution by degrading most of the organic matter before the water is released into the environment.
  • Tertiary treatment: This treatment is the final stage before water can be released into the environment. This is used only where it is needed to protect the receiving waters from excess nutrients. In tertiary treatment, the concentrations of phosphorus or nitrogen are reduced through biological or chemical processes. The treated water, called the effluent, is then disposed of in the environment.
  • Disinfection: Disinfection kills diseases-causing organisms, most commonly through chlorination. This step is only performed for water that will be used for human consumption (as opposed to being used for water the lawn, for example)


What is sludge in waste water treatment?

The substances that are removed during the wastewater treatment are called Sludge. This sludge is treated and can be used as fertilizer or in land reclamation, or will be sent for land filling. To treat sludge, waste management professionals may use anaerobic digestion, aerobic digestion or composting. Composting speeds the natural process whereby microorganisms break down complex organic matter in to simple, nutrient rich topsoil. The difference between anaerobic and aerobic digestion is that aerobic digestion happens in the presence of oxygen where as anaerobic digestion does not require oxygen. During composting, sludge is mixed with carbon before being introduced to the bacteria for digesting.


What are the disadvantages if waste water is released into the environment without treatment?

  • Environment Canada gives some examples of pollutants that can be found in waste water and the potentially harmful effects these substances can have on ecosystems and human health, if not processed properly
  • Decaying organic matter and debris can use up the dissolved oxygen in a lake so fish and other aquatic biota cannot survive
  • Chlorine compounds and inorganic chloramines can be toxic to aquatic invertebrates, algae and fishes
  • Metals such as mercury, lead, cadmium, chromium and arsenic can have acute and chronic toxic effects on species
  • Bacteria, viruses and diseases causing pathogens can pollute beaches and contaminate shell life populations


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